1. Field of the Invention
The present invention relates to exposure control of an image capturing apparatus.
2. Description of the Related Art
Conventionally, examples of a photometric value used for exposure control in an image capturing apparatus include a luminance evaluation value, and a NAM evaluation value in which a largest value of an R signal, a G signal, and a B signal of an image signal is output as an evaluation value. In ordinary automatic exposure control, on the premise that white balance has been achieved, preferable exposure control can be performed when using the NAM evaluation value as a photometric value, and, thus, the exposure control is performed usually using the NAM evaluation value as a photometric value.
FIG. 5 is a view showing the schematic configuration of a conventional image capturing apparatus. In FIG. 5, a diaphragm 192 is disposed behind a lens 191, and light that has entered the apparatus via the lens 191 and the diaphragm 192 forms an image as a subject image on an imaging plane of an imaging device 195. In the imaging device 195, the formed subject image is converted from a light signal into an image signal. The image signal obtained by the imaging device 195 is transmitted via a microcomputer 199 to a NAM evaluation value detection unit 197, and a NAM evaluation value is obtained in the NAM evaluation value detection unit 197. Then, based on the NAM evaluation value obtained from the NAM evaluation value detection unit 197, the microcomputer 199 uses an exposure control unit 193 to perform exposure control by controlling the diaphragm 192, the shutter speed of the imaging device 195, and the gain.
Meanwhile, for example, there is a conventional technique such as a method disclosed in Japanese Patent Laid-Open No. 2007-36438 for obtaining a correction amount with respect to a proper exposure value. According to this method, first, the entire screen is divided into a plurality of areas, white areas whose representative color is white are discriminated from among these areas, and the occupancy of the white areas with respect to all areas is obtained according to the number of white areas in the entire screen. Then, the obtained occupancy with respect to all areas is multiplied by a maximum exposure correction amount, and, thus, a correction amount with respect to a proper exposure value is obtained.
However, as in the case of using an image quality converting filter that has an image quality changing effect of significantly changing the tint, the white balance may be intentionally significantly shifted. Furthermore, the white balance may be intentionally significantly shifted in image quality settings by a user, or the white balance may be significantly shifted in automatic white balance correction control. If the NAM evaluation value is used when the white balance is significantly shifted from the white color contrary to ordinary cases, the influence of the tint acts, and the control is significantly shifted from ordinary proper exposure control.
Furthermore, the above-described method disclosed in Japanese Patent Laid-Open No. 2007-36438 is based on the premise that white areas are present, and is not effective when the entire screen is in chromatic colors due to the image quality changing effect or the like.
Furthermore, if the luminance evaluation value is used instead of the NAM evaluation value, the influence of the tint due to white balance shift does not act, but the performance becomes different from that in ordinary exposure control.